When the Food and Drug Administration last month approved the first defibrillator that can be implanted under the skin instead of connecting directly into the heart, it made available an advanced and less invasive treatment for US patients at risk of sudden cardiac death. The decision also gave a boost to Boston Scientific Corp. in its quest to build a more competitive product line. The company has been jockeying with rivals such as Medronic Inc. and St. Jude Medical Inc. in a cardiac rhythm management market weakened by commodity products and discounted pricing. “We haven’t had a really differentiated portfolio compared to our competitors,” Boston Scientific chief executive Hank Kucheman said in an interview at the company’s headquarters here. “This is a conscious part of our strategy to strengthen the franchise.”